(1) Field of the Invention
This invention relates to a powder surface welding method applicable for build-up welding with powder, and more particularly to a powder surface welding method suitable for surfacing an engine valve used in automobile vehicle or ship vessel and for other build-up weldings.
(2) Description of the Prior Art
For example, in the manufacture of engine valves for use in automobile vehicle, the surface welding to the valve face has frequently been performed in order to enhance the thermal resistance and abrasion resistance of the valve.
According to this surface welding method, however, it is required that the deposition to a base material such as valve or the like is complete, the composition of the deposited metal is close to the composition of the base material prior to the surface welding, the shape of weld bead is good, the yield of deposited metal is high, and the like. In order to satisfy these requirements, it is attempted to adopt plasma arc welding with powder at present.
In this powder welding with a plasma torch, a plasma working gas is supplied from a space between a bar electrode and an inner tube surrounding the bar electrode, while a surfacing powder is supplied from a space between the inner tube and an outer tube surrounding the inner tube and provided at its end with a plasma arc jetting nozzle into a plasma arc, whereby the surfacing powder is fused and surfaced onto a base material to be welded.
However, when using such a plasma torch for surfacing the base material with powder, more large-size particles tend to advance than small-size ones in the course of supplying the surfacing powder from a powder feeding means through a powder supply hose and the space between the inner and outer tubes in the torch into the plasma arc. Therefore, when the particle size distribution of the surfacing powder, welding speed, powder form or the like is unsuitable, if the supply of the surfacing powder is stopped in accordance with the completion of the surface welding, the cutting of powder supply becomes bad because the amount of small-size particles supplied at the last stage of the surface welding becomes larger as shown in FIG. 1a, and consequently a weld bead having a lower height is formed. Thus, there is obtained no desirable weld bead as shown in FIG. 1b. Therefore, if it is intended to conduct ring-like welding such as the surface welding on the engine valve face for automobile vehicle, the last fused metal composed of fine powders piles on the first surfaced portion, so that the bead form in the piled portion is bad.
Further, there are caused troubles when the arrangement between the plasma torch and the base material to be surfaced such as the distance in up and down directions, offset amount in horizontal direction or the like, the welding speed and so on are unsuitable. For instance, when the distance between the plasma torch and the base material is too large, the plasma arc becomes unstable and also the lack of fusion is caused between the deposited metal and the base material. While, when the distance is too small, the form of weld bead is poor. Alternatively, when the offset amount is unsuitable, molten metal of the surfacing powder is not well deposited on the face of the base metal to be surfaced and may fall down from the face.